Chocolate Pots Hot Springs in Yellowstone National Park are high in ferrous iron, silica and bicarbonate. The springs are contributing to
the active development of an iron formation. The microstructure of photosynthetic microbial mats in these springs was studied with
conventional optical microscopy, confocal laser scanning microscopy and transmission electron microscopy. The dominant mats at the
highest temperatures (48-54°C) were composed of Synechococcus and Chloroflexus or Pseudanabaena and Mastigocladus. At lower
temperatures (36-45°C), a narrow Oscillatoria dominated olive green cyanobacterial mats covering most of the iron deposit. Vertically
oriented cyanobacterial filaments were abundant in the top 0.5 mm of the mats. Mineral deposits accumulated beneath this surface layer.
The filamentous microstructure and gliding motility may contribute to binding the iron minerals. These activities and heavy mineral
encrustation of cyanobacteria may contribute to the growth of the iron deposit. Chocolate Pots Hot Springs provide a model for studying
the potential role of photosynthetic prokaryotes in the origin of Precambrian iron formations.

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